WO2024066086A1

WO2024066086A1 - Secondary battery and battery pack

Info

Publication number: WO2024066086A1
Application number: PCT/CN2022/141180
Authority: WO
Inventors: 陈云; 李嫚; 刘鹏; 褚春波
Original assignee: 欣旺达动力科技股份有限公司
Priority date: 2022-09-26
Filing date: 2022-12-22
Publication date: 2024-04-04
Also published as: CN115472833B; EP4407716A1; CN117276538A; CN115472833A

Abstract

Disclosed are a secondary battery and a battery pack. In the secondary battery, a negative active material layer is disposed on a current collector of a negative electrode sheet, the negative active material layer comprises a negative active material, and the negative electrode sheet has a non-faradaic specific capacitance value of 50-250 nF/g, thereby ensuring the quantity of electrochemical active sites on the surface of the negative electrode sheet, facilitating the contact between the negative electrode sheet and an electrolyte, accelerating the ion-electron conduction rate, reducing the charge transfer resistance, effectively improving the charging rate capability of the secondary battery, improving the current density of the negative electrode sheet, improving the energy density of the secondary battery, and realizing excellent dynamic performance and cycle performance of the secondary battery.

Description

Secondary batteries and battery packs

This application claims the priority of the Chinese patent application filed with the China Patent Office on September 26, 2022, with application number 202211179117.1 and invention name “Secondary Battery and Battery Pack”, the entire contents of which are incorporated by reference into this application.

Technical Field

The present application relates to the field of battery technology, and in particular to a secondary battery and a battery pack.

Background technique

With the rapid development of electric vehicles and digital electronics, secondary batteries with higher power density and energy density and suitable for fast charging and discharging are needed for electric vehicles and their electronic products. For electric vehicles, the energy density and charging time of secondary batteries, such as lithium-ion batteries, are two important technical indicators. The use of high-capacity batteries can achieve the maximum mileage, but it has a great impact on the fast charging performance.

Therefore, it is necessary to provide a secondary battery that can balance the energy density and fast charging performance of the secondary battery.

technical problem

The present application provides a secondary battery and a battery pack, which ensures the number of electrochemically active sites on the surface of the negative electrode plate by setting the non-Faraday specific capacitance of the negative electrode plate to 50nF/g~250nF/g, facilitates the contact between the negative electrode plate and the electrolyte, accelerates the ion-electron conduction rate, reduces the charge transfer resistance, and can effectively improve the charging rate performance of the secondary battery. At the same time, it can also improve the current density of the negative electrode plate and the energy density of the secondary battery, so that the secondary battery has excellent kinetic performance and cycle performance, and has good application prospects.

Technical Solutions

A first aspect of the present application provides a secondary battery, comprising a positive electrode plate, an electrolyte and a separator, and also comprising: a negative electrode plate, wherein the negative electrode plate comprises a negative electrode collector and a negative electrode active material layer disposed on at least one surface of the negative electrode collector, wherein the negative electrode active material layer comprises a negative electrode active material, and the negative electrode active material comprises graphite; wherein the non-Faraday specific capacitance of the negative electrode plate is Cdl nF/g, 50≤Cdl≤250.

In some embodiments, the resistance of the negative electrode active material layer is R mΩ, 4≤0.1×Cdl-R≤24.

In some embodiments, the value range of R is 1≤R≤15.

In some embodiments, the porosity P of the negative electrode sheet is 20% to 40%.

In some embodiments, the coating weight of the negative electrode active material layer on one surface of the negative electrode current collector is CW mg/cm ² , and 7≤CW≤12.

In some embodiments, the bulk density of the negative electrode active material is 1 g/cm ³ -2.5 g/cm ³ .

In some embodiments, the particle dispersion of the negative electrode active material is 1.5-5.

In some embodiments, the electrolyte includes a sulfur-containing additive, and the sulfur-containing additive includes at least one of formula (1) to formula (5):

Formula 1); Formula (2); Formula (3);

Formula (4); Formula (5).

In some embodiments, based on the mass of the electrolyte, the content of the sulfur-containing additive is A%, 0.05≤Cdl×A%≤12.5.

In some embodiments, based on the mass of the electrolyte, the content of the sulfur-containing additive is A%, 0.01≤A≤5.

At the same time, the second aspect of the present application also provides a battery pack, including the secondary battery as described above.

Beneficial Effects

Provided are a secondary battery and a battery pack. The secondary battery is provided with a negative electrode active material layer on a current collector of a negative electrode plate, wherein the negative electrode active material layer comprises a negative electrode active material, and the non-Faraday capacitance of the negative electrode plate is 50nF/g-250nF/g, thereby ensuring the number of electrochemically active sites on the surface of the negative electrode plate, facilitating the contact between the negative electrode plate and an electrolyte, accelerating the ion-electron conduction rate, reducing the charge transfer resistance, and being able to effectively improve the charge rate performance of the secondary battery, while also improving the current density of the negative electrode plate, thereby improving the energy density of the secondary battery, so that the secondary battery has excellent kinetic performance and cycle performance, and has good application prospects.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a cyclic voltammogram of the negative electrode sheet prepared in Example 1 of the present application at a scanning rate of 0.1 mv/s;

FIG2 is a linear scanning voltammogram of the negative electrode sheet prepared in Example 1 of the present application in the potential range of 2.6V to 2.7V and a scanning speed of 0.1mv/s;

FIG3 is a fitting curve of the scan rate-current density scatter diagram of the negative electrode sheet prepared in Example 1 of the present application.

Embodiments of the present invention

The present application provides a secondary battery and a battery pack. To make the purpose, technical solution and effect of the present application clearer and more specific, the present application is further described in detail with reference to the accompanying drawings and examples. It should be understood that the specific examples described here are only used to explain the present application and are not used to limit the present application.

In one embodiment of the present application, a secondary battery is provided, wherein the secondary battery includes a positive electrode sheet, a negative electrode sheet, a separator, an electrolyte, and a casing.

I. Negative electrode

The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. The negative electrode active material layer contains a negative electrode active material, and the negative electrode active material contains graphite.

The negative electrode sheet is a single-sided sheet or a double-sided sheet. When the negative electrode sheet is a single-sided sheet, the negative electrode active material layer is arranged on one surface of the negative electrode current collector. When the negative electrode sheet is a double-sided sheet, the negative electrode active material layer is arranged on both surfaces of the negative electrode current collector. The negative electrode sheet may also have a single-sided negative electrode sheet area and a double-sided negative electrode sheet area at the same time.

The non-Faraday ratio capacitance Cdl of the negative electrode:

The non-Faraday specific capacitance of the negative electrode is Cdl nF/g, and the value range of Cdl is 50≤Cdl≤250. Specifically, Cdl can be 50, 70, 80, 140, 160, 180, 200, 205, 210, 250 or a range consisting of any two numbers therein. The Faraday reaction in the battery refers to the process in which the oxidation state of the active material changes, and the charge moves through the double charge layer and the electrode interface to the inside of the active material; the non-Faraday reaction refers to the reaction in which the charge does not move across the electrode interface, and the charge is stored and released by the physical adsorption and detachment of ions on the electrode surface. The non-Faraday specific capacitance value Cdl nF/g reflects the number of electrochemically active sites in the negative electrode. Within a certain range, the increase in Cdl and the increase in the number of electrochemically active sites are conducive to the contact between the negative electrode active material and the electrolyte, accelerate the ion-electron conduction rate, reduce the charge transfer resistance, and can effectively improve the rate performance of the secondary battery. In some embodiments, 80≤Cdl≤250, when the non-Faraday capacitance value of the negative electrode plate is within the above range, the overall performance of the secondary battery is further improved.

In some embodiments, 80≤Cdl≤200, when the non-Faraday specific capacitance value of the negative electrode plate is within the above range, the comprehensive performance of the secondary battery can be better balanced, and the overall performance of the secondary battery is better.

Resistance R of negative electrode active material layer:

In some embodiments, the relationship between R and Cdl is: 4≤0.1×Cdl-R≤24. When the negative electrode sheet satisfies the above relationship, the ion transmission path of the negative electrode sheet is shorter, the electrochemical reaction is accelerated, and high-rate discharge performance can be achieved. When the negative electrode sheet satisfies the above relationship, the electrolyte infiltration, surface impedance, and high-rate charge and discharge performance of the negative electrode sheet are all in a more suitable state, and the expansion of the negative electrode sheet during the cycle can also be reduced.

In some embodiments, the relationship between R and Cdl is: 4≤0.1×Cdl-R≤14. When the negative electrode plate satisfies the above relationship, the internal structure of the negative electrode plate can be further optimized to improve the overall performance of the secondary battery.

In some embodiments, the resistance of the negative electrode active material layer is R mΩ, and the value range of R is 1≤R≤15. Specifically, R can be 1, 3, 4, 6, 8, 10, 12, 15, or a range consisting of any two numbers therein. The conduction characteristics of electrons mainly affect the rate performance of secondary batteries. The important factors affecting the conductivity in the negative electrode sheet of a secondary battery include the interface between the negative electrode current collector and the negative electrode active material layer, the distribution state of the conductive agent, the contact state between particles, etc. By controlling the film resistance of the negative electrode sheet (the resistance of the negative electrode active material layer), the material performance in the negative electrode sheet can be in a better state.

In some embodiments, the value range of R is 2≤R≤11. When the value of R is within this range, the conductive network inside the negative electrode plate is more complete, and the overall performance of the secondary battery is better. Moreover, if fast charging of a secondary battery is to be achieved, one of the key technologies is that the secondary battery can accept a larger charging current, which requires the polarization resistance of the battery itself to be small enough. In this application, the resistance R of the negative active material layer on the negative electrode plate is limited to 1mΩ≤R≤15mΩ, and 4≤0.1×Cdl-R≤24, that is, the membrane resistance is small enough, resulting in the polarization resistance of the secondary battery itself being small enough, and the number of active sites for electrochemical reactions is in a better range, so that the overall performance of the secondary battery is better.

In the embodiment of the present application, the test method of the resistance R of the negative electrode active material layer is:

The negative electrode sheet is cut into a disc with an area of 1540.25 ^mm2 , and the cut negative electrode sheet is placed in the middle of the probe of the diaphragm resistance tester, and then the negative electrode sheet is tested by the diaphragm resistance tester to obtain the resistance of the negative electrode active material layer of the tested negative electrode sheet; 10 negative electrode sheet samples are selected and tested by the diaphragm resistance tester respectively, and the average value of the resistance of the negative electrode active material layer of the 10 tested negative electrode sheets is obtained, which is the resistance of the negative electrode active material layer.

Porosity P of negative electrode sheet:

In some embodiments, the porosity P of the negative electrode sheet is 20% to 40%, specifically, P can be 20%, 25%, 28%, 30%, 35%, 40% or a range consisting of any two of them. When the porosity of the negative electrode sheet is within the above range, the richer the pore structure, the greater the contact area between the electrode and the electrolyte, and the shorter the transmission path of lithium ions, the faster the electrolyte infiltration, the smooth charge transmission channel provided for the solid-liquid interface, the lower the diffusion energy barrier, and the lithium ions can be quickly embedded and de-embedded into the surface of the negative electrode material, accelerating the reaction kinetics; reducing the polarization of the negative electrode surface, making the current distribution more uniform, and more negative electrode active materials participating in the acceptance of Li ⁺ at the same time during high-rate fast charging, effectively avoiding lithium precipitation on the negative electrode surface; at the same time, sufficient electrochemical reaction active sites can be provided in the electrochemical process, accelerating the non-Faraday reaction process of the negative electrode, facilitating the transfer of electrons and ions between the solid phase and the liquid phase, improving the electrochemical reaction kinetics, and ultimately improving the rate performance of the material.

The porosity of the negative electrode sheet is P = (V ₁ ‑V ₂ )/V ₁ × 100%, where V ₁ is the apparent volume of the sample and V ₂ is the actual volume of the sample. The test method can refer to GB/T 33052-2016 Porosity Determination Method.

Coating weight CW of negative electrode active material layer:

In some embodiments, the coating weight of the negative electrode active material layer on one surface of the negative electrode current collector is CW mg/cm ² , 7≤CW≤12, specifically, CW can be 7, 9, 9.5, 10, 11, 12 or a range consisting of any two numbers therein. When the non-Faraday capacitance value Cdl of the negative electrode plate is 50~250nF/g and the coating weight of the negative electrode active material layer is within the above range, while ensuring the negative electrode energy density, it is ensured that the electrolyte can be fully diffused in the negative electrode plate, avoiding the consumption of too much electrolyte for film formation, reducing the excessive consumption of lithium ions, and at the same time, the polarization in the thickness direction of the negative electrode plate is reduced, which can avoid lithium precipitation on the negative electrode surface during fast charging, and ensure the cycle life and first effect of the secondary battery. When the coating weight of the negative electrode active material layer falls within the range of 7mg/cm ² ~12mg/cm ² , the migration distance of lithium ions in the negative electrode plate is short, showing a reduced charge transfer impedance, which is beneficial to improving the battery power performance, while also improving the plate current density and improving the battery energy density.

Moreover, the change in coating weight changes the physical contact between particles in the negative electrode active material layer and the pores between particles in the negative electrode active material layer. Since the non-Faradaic reaction is a reaction in which ions are physically adsorbed and detached on the electrode surface to store and release charges, the change in the pores between particles in the negative electrode active material layer will affect the size of the non-Faradaic specific capacitance Cdl.

The coating weight of the negative electrode active material layer = (weight of the negative electrode sheet - weight of the negative electrode current collector) / area of the negative electrode sheet. When the negative electrode sheet to be tested is coated on both sides, the area of the negative electrode sheet is twice the area of the sample.

Bulk density of negative electrode active material:

In some embodiments, the bulk density of the negative electrode active material in the negative electrode active material layer is 1g/ ^cm3 ~2.5g/ ^cm3 , specifically, the bulk density can be 1, 1.5, 1.8, 2.0, 2.2, 2.5 or a range consisting of any two numbers therein. The bulk density can reflect the average density of the loose particle accumulation body including the pores inside and outside the particles and the gaps between the particles, which refers to the density obtained by dividing the mass of the powder by the volume of the container occupied by the powder. The bulk density can be controlled within the specified range according to the particle size of the raw material, calcination conditions, etc. The bulk density value reflects the uniformity of the negative electrode active material particles. The more uniform the negative electrode active material particles are, the greater the porosity of the negative electrode plate is, the liquid phase diffusion capacity of lithium ions can be improved, which is conducive to the non-Faraday reaction in the battery, resulting in an increase in the non-Faraday specific capacitance Cdl of the negative electrode plate, an increase in the number of electrochemical active points, which is conducive to the contact between the material and the electrolyte, and accelerates the ion-electron conduction rate. The charge transfer resistance is low, and the rate performance can be effectively improved.

The bulk density test of negative electrode active material can refer to GB/T 31057.1-2014 Physical properties test of granular materials Part 1 Bulk density measurement.

Particle dispersion of negative electrode active material:

In some embodiments, the particle dispersion of the negative electrode active material is 1.5 to 5, specifically, the particle dispersion can be 1.5, 1.8, 2.0, 2.2, 2.5, 2.7, 3.0, 3.2, 3.5, 3.8, 4.0, 4.2, 4.5, 4.8, 5 or a range consisting of any two numbers therein. The particle dispersion can reflect the concentration and dispersion of the content of various particle size components. The dispersion affects the powder dispersion effect during the stirring process, so that the obtained slurry has good uniformity and stability, is not easy to settle, can effectively regulate the uniformity of the coating weight of the negative electrode sheet, enhance the stability of the negative electrode sheet, and further regulate the non-Faraday process, improve the transmission efficiency of lithium ions between the positive and negative electrodes, and improve the rate performance of the secondary battery.

In some embodiments, the particle dispersion of the negative electrode active material is 1.5 to 3.5. When the particle dispersion of the negative electrode active material is within this range, the negative electrode slurry has better uniformity, the formed negative electrode sheet has better apparent performance, and the comprehensive performance of the secondary battery is better.

The particle dispersion of the negative electrode active material can be tested by a laser particle size analyzer. The particle dispersion of the negative electrode active material = (Dv99-Dv10)/Dv50.

Test method for non-Faraday ratio capacitance Cdl value:

In some embodiments, referring to FIG. 1 to FIG. 3 , a method for testing the non-Faraday capacitance Cdl value of a negative electrode plate includes the following steps:

S1. Confirmation of non-Faraday potential interval: Assemble the negative electrode into a half-cell (referred to as buckle) for cyclic voltammetry (CV) test, where the voltage range is 0.005~3.0V and the scan rate is 0.1~1mV/s. Figure 1 shows the cyclic voltammogram of the negative electrode at a scan rate of 0.1mv/s. As can be seen from Figure 1, the cyclic voltammogram includes two curves, which only represent two scans at the same scan rate. Because the first scan is irreversible, the peak positions of the two scans will be different. The non-Faraday potential interval is the straight section of the curve, that is, the 1.5V~3V interval;

S2, non-Faraday interval cathode scanning: select a potential interval from the potential interval of 1.5V~3V confirmed in step S1 to perform linear scanning voltammetry (LSV) test, the scanning direction is from high potential to low potential, and the voltage-current curve is collected, wherein the selected potential interval is 2.6~2.7V, and the scanning speed is 0.05-5mV/s. FIG2 shows the linear scanning voltammogram at a scanning speed of 0.1mv/s;

Furthermore, the median U V, U=2.65, in the potential interval of 2.6-2.7 V was selected at a scan rate of 0.1 mv/s to obtain the corresponding current value, and the current density value J1 was calculated according to the corresponding active material mass, in units of A/g; within the scan rate range of 0.05-5 mV/s, the current values corresponding to the median U at different scan rates were selected, and different current density values were calculated to obtain a scan rate-current density scatter plot;

S3. Calculation of non-Faraday capacitance value: According to the scan rate-current density scatter plot collected in step S2, a linear function is fitted, and the slope K of the linear function is the non-Faraday capacitance Cdl value of the negative electrode plate in the cathode scanning direction.

The assembly method of the buckle battery is as follows: the obtained negative electrode sheet is dried, cut into small discs, weighed, and transferred to a vacuum oven, dried at 100°C for 8 hours, and then transferred to a glove box filled with argon for half-cell assembly. The assembly method is the conventional assembly method in the field. The non-Faraday potential interval is confirmed using the cyclic voltammetry curve, and then the current value is measured using the linear voltammetry scanning curve within the potential interval to calculate the non-Faraday specific capacitance Cdl value.

Negative current collector:

In some embodiments, the negative electrode current collector includes, but is not limited to, metal foil, metal cylinder, metal strip, metal plate, metal film, metal mesh, stamped metal, foamed metal, etc. In some embodiments, the negative electrode current collector is metal foil. In some embodiments, the negative electrode current collector is aluminum foil or copper foil. As used herein, the term "copper foil" includes copper alloy foil.

In some embodiments, the negative electrode current collector is a conductive resin. In some embodiments, the conductive resin includes a film obtained by evaporating copper on a polypropylene film.

Negative active material layer:

The negative electrode active material layer may be one or more layers, and each layer of the multiple negative electrode active material layers may contain the same or different negative electrode active materials. The negative electrode active material is any material that can reversibly embed and deintercalate metal ions such as lithium ions. In some embodiments, the chargeable capacity of the negative electrode active material is greater than the discharge capacity of the positive electrode active material to prevent lithium metal from being precipitated on the negative electrode sheet during charging.

In some embodiments, the thickness of the negative electrode active material layer refers to the thickness of the negative electrode active material layer coated on a single side of the negative electrode current collector. In some embodiments, the thickness of the single-sided negative electrode active material layer is 15 μm or more. In some embodiments, the thickness of the single-sided negative electrode active material layer is 20 μm or more. In some embodiments, the thickness of the single-sided negative electrode active material layer is 30 μm or more. In some embodiments, the thickness of the single-sided negative electrode active material layer is 150 μm or less. In some embodiments, the thickness of the single-sided negative electrode active material layer is 120 μm or less. In some embodiments, the thickness of the single-sided negative electrode active material layer is 100 μm or less. In some embodiments, the thickness of the negative electrode active material layer is within the range consisting of any two of the above values. When the thickness of the negative electrode active material layer is within the above range, the electrolyte can penetrate into the vicinity of the negative electrode current collector interface, improving the charge and discharge characteristics of the secondary battery at high current density; at the same time, the volume ratio of the negative electrode current collector to the negative electrode active material is within an appropriate range, which can ensure the capacity of the secondary battery.

In some embodiments, the negative electrode active material layer includes a negative electrode active material, a conductive agent, a binder, and a dispersant.

Negative active material:

In some embodiments, the negative electrode active material includes one or more of artificial graphite, natural graphite, soft carbon, hard carbon, amorphous carbon, carbon nanotubes, and mesophase carbon microspheres.

Conductive agent:

In some embodiments, the conductive agent includes one or more of carbon black, graphite, carbon fiber, carbon nanotube or graphene, preferably carbon black.

Binder:

The binder can improve the bonding between the negative electrode active materials. The type of the binder is not particularly limited, as long as it is a material that is stable to the electrolyte or the solvent used in the manufacture of the electrode. In some embodiments, the binder includes sodium carboxymethyl cellulose and styrene-butadiene rubber.

Dispersant:

In some embodiments, the dispersant comprises diethylhexanol, which is an environmentally friendly organic compound with low price and wide source. It has low surface tension and is easy to adsorb and spread on the liquid surface. The material is subjected to shear and friction by mechanical force, and there is internal friction between the particles. Under the action of various forces, the raw material particles tend to be highly dispersed, making the slurry more uniform and having a good dispersion effect. The prepared dry electrode has a uniform thickness, avoiding wrinkling and other problems that affect the electrical performance, enhancing the stability of the electrode, improving the transmission efficiency of lithium ions between the positive and negative electrodes, reducing electrochemical polarization, accelerating the non-Faraday reaction process, and meeting the requirements of power battery rate performance and cycle life.

II. Electrolyte

Formula 1); Formula (2); Formula (3);

Formula (4); Formula (5).

Based on the mass of the electrolyte, the content of the sulfur-containing additive is A%, 0.05≤Cdl×A%≤12.5, preferably 0.01≤A≤5, and specifically, A can be 0.1, 0.5, 0.7, 1, 1.5, 1.8, 2.2, 2.7, 3, 4 or a range consisting of any two numbers therein. When the non-Faraday capacitance of the negative electrode plate is 50nF/g~250nF/g and the electrolyte contains the sulfur-containing additive, the sulfur-containing additive has high stability and high ion conductivity, and the interface film formed by the sulfur-containing additive is mainly composed of organic sulfide, with low impedance, which improves the lithium ion conductivity, thereby extending the life of the secondary battery, reducing the side reactions of the electrolyte components themselves and the reactions between them and the negative electrode materials, improving the migration rate of lithium ions, reducing the impedance of the SEI film, thereby reducing the impedance of the secondary battery, reducing polarization, and improving the charge and discharge efficiency. At the same time, lithium plating of the negative electrode plate is avoided to cause internal short circuit of the secondary battery. The combined effect of the negative electrode plate and the electrolyte helps to improve the battery cycle performance. An appropriate amount of sulfur-containing additives can improve the lithium ion conduction efficiency, accelerate the non-Faraday reaction process, and improve the ability to charge quickly at a high rate. In some embodiments of the present application, 0.1≤A≤4. In some embodiments of the present application, 0.5≤A≤3. When the mass percentage of the sulfur-containing additive is within the above range, and the non-Faraday specific capacitance of the negative electrode plate is 50nF/g~250nF/g, an electrochemical reaction is carried out between the two, and the protective film formed is relatively complete and the density of the film is appropriate, which reduces the polarization phenomenon and improves the cycle performance of the secondary battery.

Lithium salts:

In some embodiments, the lithium salt includes at least one of lithium hexafluorophosphate, organic lithium borate, lithium perchlorate, and sulfonyl imide lithium salts. The content of the lithium salt is not particularly limited as long as it does not impair the effects of the present application.

III. Positive electrode

The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector.

Positive electrode active material layer:

The positive electrode active material layer may be one layer or multiple layers. Each layer of the multiple layers of positive electrode active material may contain the same or different positive electrode active materials. The positive electrode active material is any material that can reversibly embed and de-embed metal ions such as lithium ions.

In some embodiments, the positive electrode active material includes one or more of lithium manganese oxide, lithium iron phosphate (LFP) and a ternary material.

In some embodiments, the positive electrode active material includes a ternary material, and the ternary material may include lithium nickel cobalt manganese oxide and/or lithium nickel cobalt aluminum oxide.

In some embodiments, the positive electrode active material comprises lithium nickel cobalt manganese oxide, and the content of nickel element is greater than or equal to 0.5, based on the molar ratio of nickel element, cobalt element and manganese element being 1.

In some embodiments, the positive electrode active material comprises lithium nickel cobalt manganese oxide, and the content of nickel element is less than or equal to 0.85, based on the molar ratio of nickel element, cobalt element and manganese element being 1.

In some embodiments, the positive electrode active material comprises a doping element and/or a coating element. There is no particular requirement for the doping element and/or the coating element, as long as the positive electrode active material can be made more stable.

In addition, the positive electrode active material also includes a positive electrode conductor, a positive electrode binder and a solvent.

Positive electrode conductive agent:

There is no limitation on the type of positive electrode conductive agent, and any known conductive agent can be used. Examples of positive electrode conductive agents may include, but are not limited to, graphite such as natural graphite and artificial graphite; carbon black such as acetylene black; carbon materials such as amorphous carbon such as needle coke; carbon nanotubes; graphene, etc. The above positive electrode conductive agents may be used alone or in any combination.

Positive electrode binder:

The type of the positive electrode binder used in the production of the positive electrode active material layer is not particularly limited, and in the case of a coating method, any material may be used as long as it is soluble or dispersible in a liquid medium used in the production of the electrode. Examples of positive electrode binders may include, but are not limited to, one or more of the following: resin-based polymers such as polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, and nitrocellulose; rubber-like polymers such as styrene-butadiene rubber (SBR), nitrile rubber (NBR), fluororubber, isoprene rubber, and ethylene-propylene rubber; thermoplastic elastomer-like polymers such as styrene-butadiene-styrene block copolymers or their hydrides, ethylene-propylene-diene terpolymers (EPDM), styrene-ethylene-butadiene-ethylene copolymers, styrene-isoprene-styrene block copolymers or their hydrides; soft resin-like polymers such as syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene-vinyl acetate copolymers, and propylene-α-olefin copolymers; fluorine-based polymers such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene, fluorinated polyvinylidene fluoride, and polytetrafluoroethylene-ethylene copolymers; polymer compositions having ion conductivity of alkali metal ions (especially lithium ions), etc. The above positive electrode binders can be used alone or in any combination.

Solvent:

There is no restriction on the type of solvent used to form the positive electrode slurry, as long as it is a solvent that can dissolve or disperse the positive electrode active material, the positive electrode conductor, and the positive electrode binder. Examples of solvents used to form positive electrode slurries may include any of aqueous solvents and organic solvents. Examples of aqueous media may include, but are not limited to, water, mixed media of alcohol and water, etc. Examples of organic media may include, but are not limited to, solvents such as diethylenetriamine, N, N-dimethylaminopropylamine, diethyl ether, propylene oxide, tetrahydrofuran (THF), N-methylpyrrolidone (NMP), dimethylformamide, dimethylacetamide, hexamethylphosphoramide, and dimethyl sulfoxide.

Positive current collector:

There is no particular limitation on the type of positive electrode current collector, which can be any material known to be suitable for use as a positive electrode current collector. Examples of positive electrode current collectors may include, but are not limited to, metal materials such as aluminum, stainless steel, nickel plating, titanium, tantalum, etc.; carbon materials such as carbon cloth and carbon paper; composite materials formed by polymers and metal layers. In some embodiments, the positive electrode current collector is a metal material. In some embodiments, the positive electrode current collector is aluminum.

There is no particular restriction on the form of the positive electrode current collector. When the positive electrode current collector is a metal material, the form of the positive electrode current collector may include, but is not limited to, metal foil, metal cylinder, metal strip roll, metal plate, metal foil, metal plate mesh, stamped metal, foamed metal, etc. When the positive electrode current collector is a carbon material, the form of the positive electrode current collector may include, but is not limited to, carbon plate, carbon film, carbon cylinder, etc. In some embodiments, the positive electrode current collector is a metal foil. In some embodiments, the metal foil is mesh-shaped. There is no particular restriction on the thickness of the metal foil. In some embodiments, the thickness of the metal foil is greater than 1 μm, greater than 3 μm, or greater than 5 μm. In some embodiments, the thickness of the metal foil is less than 1 mm, less than 50 μm, or less than 20 μm. In some embodiments, the thickness of the metal foil is within the range composed of any two of the above values.

IV. Isolation membrane

In order to prevent short circuit, a separator is usually provided between the positive electrode and the negative electrode. In this case, the electrolyte of the present application is usually used by infiltrating the separator.

V. Application

The present application also provides a battery pack, including the above-mentioned secondary battery. As a typical application, the battery pack can be used for, but not limited to, electric toys, electric tools, battery vehicles, electric vehicles, energy storage equipment, ships, spacecraft, etc.

The following is a description of the method for preparing the secondary battery provided by the present application in conjunction with specific embodiments:

Example 1

Preparation of positive electrode:

The positive electrode active material, positive electrode conductive agent, positive electrode binder and solvent are prepared into positive electrode slurry according to the formula. By mass percentage, the positive electrode active material NCM811: positive electrode conductive agent carbon black: positive electrode binder polyvinylidene fluoride (PVDF) is 96:2:2, and the solvent is N-methylpyrrolidone (NMP); the prepared positive electrode slurry is evenly coated on the two surfaces of the positive electrode collector (aluminum foil), and then dried at 120°C to obtain a positive electrode sheet. The thickness of the positive electrode sheet is controlled by roller pressing, and its compaction density is controlled at 3.5g/ ^cm3 .

Preparation of negative electrode sheet:

First, a negative electrode slurry is prepared, wherein the negative electrode active material in the negative electrode slurry includes artificial graphite, conductive carbon black, sodium carboxymethyl cellulose, styrene butadiene rubber and diethyl hexanol, and the mass ratio of artificial graphite, conductive carbon black, sodium carboxymethyl cellulose, styrene butadiene rubber and diethyl hexanol is 96.5:1.2:1.2:1.0:0.1. Among them, the bulk density of artificial graphite is 1.5 g/ ^cm3 , and the dispersion is 2.0.

The specific preparation method of the negative electrode slurry is as follows: add 50% artificial graphite, conductive agent carbon black, 50% artificial graphite and 70% sodium carboxymethyl cellulose to a double planetary stirring tank in order, adjust the stirring tank revolution speed to 20r/min, the rotation speed to 800r/min, stir at low speed for 30min to mix evenly, add part of deionized water and stir for 1h to form a first mixed powder; scrape and turn the bottom, add the remaining 30% sodium carboxymethyl cellulose and deionized water to the first mixed powder, the revolution speed is 25r/min, the rotation speed is 2500r/min, and the high-speed stirring is 90min. The vacuum degree is controlled at -0.085MPa in this process to disperse the slurry to form a second mixed solution; scrape and turn the bottom, add the binder styrene-butadiene rubber and 50% diethyl hexanol to the second mixed solution, the revolution speed is 20r/min, and the rotation speed is 2500r/min. The mixture was stirred at a low speed of 500 r/min for 30 min, and the vacuum degree was controlled at -0.085 MPa during the process to form a third mixed solution; 50% diethylhexanol was finally added to the third mixed solution, the orbital speed was 10 r/min, the self-rotation speed was 100 r/min, and the mixture was stirred for 30 min, and the vacuum degree was controlled at -0.085 MPa during the process to form a fourth mixed solution; finally, deionized water was added to further adjust the viscosity of the slurry, the orbital speed was 25 r/min, and the self-rotation speed was 300 r/min and stirred for 30 min, which was more conducive to uniform dispersion of the slurry. The vacuum degree was controlled at -0.085 MPa during the process, and the slurry was qualified when the viscosity of the slurry was between 2000 mPa·s and 3500 mPa·s. The self-rotation speed was 200 r/min for defoaming treatment and then stored, and the material was discharged through a 200-mesh sieve to prepare the negative electrode slurry. The prepared negative electrode slurry is evenly coated on one or both sides of the negative electrode current collector copper foil, and the coating surface density is controlled at 9.5 mg/ ^cm2 . After baking and drying, the negative electrode sheet is rolled and cut into pieces. The baking temperature is 90℃~110℃, the drying time is 24h, and the porosity P of the negative electrode sheet after rolling is 30%.

Among them, the preparation of negative electrode slurry is mainly to fully mix the active material with the inactive components to give full play to the electrochemical properties of the electrode sheet, including the uniformity of the slurry, avoiding agglomeration and sedimentation, ensuring the controllable coating weight of the electrode sheet, and the matching of the active material with the inactive components, ensuring the movement of the lithium-ion battery between the positive and negative electrodes, and improving the overall electrochemical performance of the battery.

Preparation of electrolyte:

Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (EDC) were mixed in a volume ratio of 1:1:1, and then 1 mol/L LiPF ₆ was added and mixed evenly to prepare an electrolyte.

Preparation of secondary batteries:

The negative electrode sheet and the positive electrode sheet prepared by the above steps are dried and then used together with the isolation film to prepare a wound battery cell using a winding machine, the positive electrode aluminum tab and the negative electrode copper nickel-plated tab are welded on the battery cell, and the welded battery cell is placed in a punched aluminum-plastic film for packaging, and the isolation film is a PP film; after liquid injection and formation to constant capacity, a secondary battery is obtained.

Non-Faraday ratio capacitance Cdl test of negative electrode:

Step 1: disassemble the secondary battery in a glove box to obtain the negative electrode overhang area electrode sheet, soak it in DMC (dimethyl carbonate) solution, cut it and assemble it with a metal lithium sheet into a button half-cell; or take the negative electrode electrode sheet that has not been assembled into a battery and cut it and assemble it with a metal lithium sheet into a button half-cell;

Step 2, perform CV test on the button half-cell in the voltage range of 0.005V~3.0V, with a scan rate of 0.1mV/s, and confirm that the non-Faraday potential interval is 2.6V~2.7V;

Step 3, then perform LSV test from 2.7V to 2.6V, with scan rates of 0.1mV/s, 0.2mV/s, 0.5mV/s, 1mV/s, 2mV/s, respectively, select the median of the potential interval of 2.65V, and obtain the corresponding current values of -4.32E-07A, -5.50E-07A, -1.15E-06A, -2.08E-06 A, -3.80E-06 A, and calculate the current density values of -1.25E-05A/g, -2.08E-05A/g, -4.75E-05A/g, -8.87E-05A/g, -1.64E-04A/g according to the corresponding active material mass of 0.02175g;

Step 4, draw a scan rate-current density scatter plot based on the scan rate and current density values obtained in step 3, and fit a linear function. The slope of the linear function -8E-05 is the non-Faraday specific capacitance of the negative electrode in the cathode scanning direction, which is 80nF/g.

The negative electrode sheet overhang area refers to the portion of the negative electrode sheet that exceeds the positive electrode sheet in length and width directions.

Example 2

A secondary battery was prepared according to the method of Example 1, except for the following differences, the rest was the same as Example 1:

The coating weight CW mg/cm ² of the negative electrode sheet during coating is 7 mg/cm ² , the porosity P of the negative electrode sheet is 25%, the resistance R mΩ of the negative electrode active material layer is 3 mΩ, and the non-Faraday capacitance Cdl nF/g of the negative electrode sheet is 140 nF/g.

Example 3

The coating weight CW mg/cm ² of the negative electrode sheet during coating is 12 mg/cm ² , the porosity P of the negative electrode sheet is 25%, the resistance R mΩ of the negative electrode active material layer is 15 mΩ, and the non-Faraday capacitance Cdl nF/g of the negative electrode sheet is 200 nF/g.

Example 4

The coating weight CW mg/cm ² of the negative electrode sheet during coating is 9 mg/cm ² , the porosity P of the negative electrode sheet is 20%, the resistance R mΩ of the negative electrode active material layer is 1 mΩ, and the non-Faraday capacitance Cdl nF/g of the negative electrode sheet is 250 nF/g.

Example 5

The coating weight CW mg/cm ² of the negative electrode sheet during coating is 9 mg/cm ² , the porosity P of the negative electrode sheet is 40%, the resistance R mΩ of the negative electrode active material layer is 8 mΩ, and the non-Faraday capacitance Cdl nF/g of the negative electrode sheet is 50 nF/g.

Example 6

Example 7

The coating weight CW mg/cm ² of the negative electrode sheet during coating is 10.5 mg/cm ² , the porosity P of the negative electrode sheet is 26%, the resistance R mΩ of the negative electrode active material layer is 12 mΩ, and the non-Faraday capacitance Cdl nF/g of the negative electrode sheet is 230 nF/g.

Example 8

The coating weight CW mg/cm ² of the negative electrode sheet during coating is 9 mg/cm ² , the porosity P of the negative electrode sheet is 20%, the resistance R mΩ of the negative electrode active material layer is 2 mΩ, and the non-Faraday capacitance Cdl nF/g of the negative electrode sheet is 250 nF/g.

Example 9

The coating weight CW mg/cm ² of the negative electrode sheet during coating is 10 mg/cm ² , the porosity P of the negative electrode sheet is 40%, the resistance R mΩ of the negative electrode active material layer is 2 mΩ, and the non-Faraday capacitance Cdl nF/g of the negative electrode sheet is 50 nF/g.

Example 10

The coating weight CW mg/cm ² of the negative electrode sheet during coating is 9.2 mg/cm ² , the porosity P of the negative electrode sheet is 30%, the resistance R mΩ of the negative electrode active material layer is 2.5 mΩ, and the non-Faraday capacitance Cdl nF/g of the negative electrode sheet is 65 nF/g.

Embodiment 11

The coating weight CW mg/cm ² of the negative electrode sheet during coating is 9.1 mg/cm ² , the porosity P of the negative electrode sheet is 25%, the resistance R mΩ of the negative electrode active material layer is 6.3 mΩ, and the non-Faraday capacitance Cdl nF/g of the negative electrode sheet is 150 nF/g.

Example 12

The coating weight CW mg/cm ² of the negative electrode sheet during coating is 9.6 mg/cm ² , the porosity P of the negative electrode sheet is 30%, the resistance R mΩ of the negative electrode active material layer is 9.5 mΩ, and the non-Faraday capacitance Cdl nF/g of the negative electrode sheet is 150 nF/g.

Example 13

The coating weight CW mg/cm ² of the negative electrode sheet during coating is 10 mg/cm ² , the porosity P of the negative electrode sheet is 22%, the resistance R mΩ of the negative electrode active material layer is 11 mΩ, and the non-Faraday capacitance Cdl nF/g of the negative electrode sheet is 150 nF/g.

Comparative Example 1

The coating weight CW mg/cm ² of the negative electrode sheet during coating is 5 mg/cm ² , the porosity P of the negative electrode sheet is 42%, the resistance R mΩ of the negative electrode active material layer is 10 mΩ, and the non-Faraday capacitance Cdl nF/g of the negative electrode sheet is 260 nF/g.

Comparative Example 2

The coating weight CW mg/cm ² of the negative electrode sheet during coating is 14 mg/cm ² , the porosity P of the negative electrode sheet is 18%, the resistance R mΩ of the negative electrode active material layer is 12 mΩ, and the non-Faraday capacitance Cdl nF/g of the negative electrode sheet is 45 nF/g.

The performance test process and test results of the secondary battery provided in the embodiment of the present application are as follows:

(1) Dynamic performance test

Place the secondary battery at 25°C for 30 minutes, fully charge at xC and fully discharge at 1C for 10 times, then fully charge at xC, disassemble the negative electrode, and observe the lithium deposition on the surface of the negative electrode. If there is no lithium deposition on the negative electrode surface, increase the charging rate xC by 0.1C and test again until lithium deposition occurs on the negative electrode surface. Stop the test. The charging rate xC minus 0.1C is the maximum charging rate of the battery.

(2) Energy density test

The secondary battery is placed at 25°C for 30 minutes, fully charged at 1C and fully discharged at 1C, and the actual discharge energy is recorded; the lithium-ion battery is weighed with an electronic balance; the ratio of 1C actual discharge energy to weight is the actual energy density of the secondary battery. Among them, when the actual energy density is less than 80% of the target energy density, the actual energy density of the secondary battery is considered to be very low; when the actual energy density is greater than or equal to 80% of the target energy density and less than 95%, the actual energy density of the secondary battery is considered to be low; when the actual energy density is greater than or equal to 95% of the target energy density and less than 105%, the actual energy density of the secondary battery is considered to be moderate; when the actual energy density is greater than or equal to 105% of the target energy density and less than 120%, the actual energy density of the secondary battery is considered to be high; when the actual energy density is 120% of the target energy density, the actual energy density of the secondary battery is considered to be very high.

(3) Cyclic performance test

The secondary battery was placed at 25°C for 30 min, discharged at 1C constant current, placed for 10 min, charged at 1C constant current and constant voltage, placed for 10 min, and a full charge and discharge cycle test was performed, and the capacity retention rate after 1000 cycles was recorded.

The performance test results of the secondary batteries provided in Examples 1 to 13 and the performance test results of the secondary batteries provided in Comparative Examples 1 and 2 are shown in Table 1.

Table 1

	CW CW	P (%) P (%)	R R	Cdl Cdl	0.1×Cdl-R 0.1×Cdl-R	能量密度比（%） Energy density ratio (%)	容量保持率 (%) Capacity retention rate (%)	动力学性能（C） Dynamic performance (C)
实施例1 Example 1	9.5 9.5	30 30	4.6 4.6	80 80	3.4 3.4	125 125	95 95	2.5 2.5
实施例2 Example 2	8 8	30 30	5 5	100 100	5 5	112 112	94 94	3.5 3.5
实施例3 Example 3	7.5 7.5	35 35	2 2	120 120	10 10	108 108	95 95	3.8 3.8
实施例4 Example 4	7 7	25 25	3 3	140 140	11 11	105 105	92 92	4 4
实施例5 Example 5	8.5 8.5	35 35	2.5 2.5	160 160	13.5 13.5	118 118	92 92	3.7 3.7
实施例6 Example 6	12 12	25 25	15 15	200 200	5 5	133 133	96 96	2 2
实施例7 Example 7	10.5 10.5	26 26	12 12	230 230	11 11	128 128	94 94	2.4 2.4
实施例8 Example 8	9.2 9.2	30 30	3 3	250 250	22 twenty two	110 110	92 92	2.6 2.6
实施例9 Example 9	10 10	40 40	2 2	50 50	3 3	115 115	91 91	2.4 2.4
实施例10 Example 10	9.2 9.2	30 30	2.5 2.5	65 65	4 4	120 120	91 91	2.5 2.5
实施例11 Example 11	9.1 9.1	25 25	6.3 6.3	150 150	8.7 8.7	122 122	93 93	3.1 3.1
实施例12 Example 12	9.6 9.6	30 30	9.5 9.5	150 150	5.5 5.5	125 125	91 91	2.6 2.6
实施例13 Example 13	10 10	22 twenty two	11 11	150 150	4 4	124 124	91 91	2.2 2.2
对比例1 Comparative Example 1	5 5	42 42	10 10	260 260	16 16	75 75	85 85	1.2 1.2
对比例2 Comparative Example 2	14 14	18 18	12 12	45 45	-7.5 -7.5	85 85	86 86	1.5 1.5

As can be seen from Table 1, the secondary batteries provided in Examples 1 to 13 have better kinetic performance, higher or very high energy density, and a capacity retention rate of cycle performance of up to 91% to 96%. This allows the secondary batteries provided in the embodiments of the present application to be quickly charged at a large rate while having a higher or even very high energy density and excellent cycle performance (the capacity retention rate of cycle performance is as high as 91% to 96%).

In Comparative Example 1, the coating amount of the negative electrode active material layer is 5 mg/cm ² , the porosity P of the negative electrode sheet is 42%, and the non-Faraday capacitance Cdl value of the negative electrode sheet is as high as 260 nF/g, resulting in poor kinetic performance of the secondary battery provided in Comparative Example 1, very low energy density, and only 85% capacity retention rate after the cycle performance test. In Comparative Example 2, the coating amount of the negative electrode active material layer is 14 mg/cm ² , the porosity P of the negative electrode sheet is 18%, and the non-Faraday capacitance Cdl value of the negative electrode sheet is 260. The kinetic performance of the secondary battery provided in Comparative Example 2 is poor, and although it has a higher energy density, the capacity retention rate after the cycle performance test is only 86%.

Therefore, it is difficult for the secondary batteries provided in Comparative Examples 1 and 2 to be quickly charged at a high rate while maintaining a high energy density and excellent cycle performance.

Embodiment 14

A secondary battery was prepared according to the method of Example 1. The coating weight CW mg/cm ² of the negative electrode active material layer was 9 mg/cm ² , and the porosity P was 35%. The rest was the same as Example 1 except for the following differences:

The bulk density of the negative electrode active material is 1.5 g/cm ³ , the particle dispersion of the negative electrode active material is 2, and the non-Faraday specific capacitance Cdl nF/g of the negative electrode plate is 200 nF/g.

Embodiment 15

A secondary battery was prepared according to the method of Example 1. The coating weight CW of the negative electrode active material layer was 9 mg/cm ² , and the porosity P was 35%. The rest was the same as Example 1 except for the following differences:

The bulk density of the negative electrode active material is 1 g/cm ³ , the particle dispersion of the negative electrode active material is 1.5, and the non-Faraday specific capacitance Cdl nF/g of the negative electrode plate is 240 nF/g.

Example 16

The bulk density of the negative electrode active material is 1.8 g/cm ³ , the particle dispersion of the negative electrode active material is 2.5, and the non-Faraday specific capacitance Cdl nF/g of the negative electrode plate is 180 nF/g.

Embodiment 17

The bulk density of the negative electrode active material is 2 g/cm ³ , the particle dispersion of the negative electrode active material is 3, and the non-Faraday specific capacitance Cdl nF/g of the negative electrode plate is 150 nF/g.

Embodiment 18

The bulk density of the negative electrode active material is 2.2 g/cm ³ , the particle dispersion of the negative electrode active material is 4, and the non-Faraday specific capacitance Cdl nF/g of the negative electrode plate is 100 nF/g.

Embodiment 19

The bulk density of the negative electrode active material is 2.5 g/cm ³ , the particle dispersion of the negative electrode active material is 5, and the non-Faraday specific capacitance Cdl nF/g of the negative electrode sheet is 60 nF/g.

The performance test results of the secondary batteries provided in Examples 14 to 19 are shown in Table 2.

Table 2

	松密度（g/cm3） Bulk density (g/cm3)	颗粒分散度 Particle dispersion	Cdl Cdl	动力学性能（C） Dynamic performance (C)	容量保持率（%） Capacity retention rate (%)
实施例14 Example 14	1.5 1.5	2 2	200 200	3.5 3.5	95 95
实施例15 Example 15	1 1	1.5 1.5	240 240	4 4	94 94
实施例16 Example 16	1.8 1.8	2.5 2.5	180 180	3.1 3.1	92 92
实施例17 Example 17	2 2	3 3	150 150	2.6 2.6	93 93
实施例18 Example 18	2.2 2.2	4 4	100 100	2.3 2.3	94 94
实施例19 Example 19	2.5 2.5	5 5	60 60	2 2	92 92

As can be seen from Table 2, in the kinetic performance test results of the secondary batteries provided in Examples 14 to 19, the kinetic performance is as high as 2C to 4C, so that the secondary batteries provided in Examples 14 to 19 can be quickly charged at a large rate, while the secondary batteries have a high or even very high energy density and excellent cycle performance.

Embodiment 20

The sulfur-containing additive shown in formula (4) is selected as the sulfur-containing additive in the electrolyte, and the content of the sulfur-containing additive in the electrolyte is 0.05%.

Embodiment 21

The sulfur-containing additive shown in formula (4) is selected as the sulfur-containing additive in the electrolyte, and the content of the sulfur-containing additive in the electrolyte is 0.1%.

Embodiment 22

The sulfur-containing additive shown in formula (4) is selected as the sulfur-containing additive in the electrolyte, and the content of the sulfur-containing additive in the electrolyte is 0.5%.

Embodiment 23

The sulfur-containing additive shown in formula (4) is selected as the sulfur-containing additive in the electrolyte, and the content of the sulfur-containing additive in the electrolyte is 1%.

Embodiment 24

The sulfur-containing additive shown in formula (4) is selected as the sulfur-containing additive in the electrolyte, and the content of the sulfur-containing additive in the electrolyte is 2%.

Embodiment 25

The sulfur-containing additive shown in formula (4) is selected as the sulfur-containing additive in the electrolyte, and the content of the sulfur-containing additive in the electrolyte is 5%.

Embodiment 26

The sulfur-containing additive shown in formula (3) is selected as the sulfur-containing additive in the electrolyte, and the content of the sulfur-containing additive in the electrolyte is 0.1%.

Embodiment 27

A secondary battery was prepared according to the method of Example 1, except for the following differences, which were the same as Example 1:

The sulfur-containing additive shown in formula (3) is selected as the sulfur-containing additive in the electrolyte, and the content of the sulfur-containing additive in the electrolyte is 1%.

Embodiment 28

A combination of formula (3) and (4) is selected as the sulfur-containing additive in the electrolyte. The content of the sulfur-containing additive represented by formula (3) in the electrolyte is 0.5%, and the content of the sulfur-containing additive represented by formula (4) is 0.5%.

Embodiment 29

A combination of formula (1) and (3) is selected as the sulfur-containing additive in the electrolyte. The content of the sulfur-containing additive represented by formula (1) in the electrolyte is 0.3%, and the content of the sulfur-containing additive represented by formula (3) is 0.5%.

The performance test results of the secondary batteries provided in Examples 14 to 19 are shown in Table 3.

table 3

	含硫添加剂种类 Types of sulfur-containing additives	含硫添加剂含量（%） Sulfur additive content (%)	动力学性能（C） Dynamic performance (C)	容量保持率（%） Capacity retention rate (%)
实施例20 Example 20	式（4） Formula (4)	0.05 0.05	2.6 2.6	95 95
实施例21 Example 21	式（4） Formula (4)	0.1 0.1	2.8 2.8	95 95
实施例22 Example 22	式（4） Formula (4)	0.5 0.5	3.1 3.1	96 96
实施例23 Example 23	式（4） Formula (4)	1 1	3.6 3.6	97 97
实施例24 Example 24	式（4） Formula (4)	2 2	4 4	95 95
实施例25 Example 25	式（4） Formula (4)	5 5	2.9 2.9	94 94
实施例26 Example 26	式（3） Formula (3)	0.1 0.1	2.9 2.9	95 95
实施例27 Example 27	式（3） Formula (3)	1 1	3.6 3.6	96 96
实施例28 Example 28	式（3）+式（4） Formula (3) + Formula (4)	0.5+0.5 0.5+0.5	3.8 3.8	96 96
实施例29 Example 29	式（1）+式（3） Formula (1) + Formula (3)	0.3+0.5 0.3+0.5	3.7 3.7	95 95

It can be seen from Table 3 that in the kinetic performance test results of the secondary batteries provided in Examples 20 to 29, the kinetic performance is as high as 2.6C to 4C, so that the secondary batteries provided in Examples 20 to 29 can be quickly charged at a large rate, while the secondary batteries have a high or even very high energy density and have excellent cycle performance (the capacity retention rate after the cycle performance test is as high as 94% to 96%).

The secondary battery and battery pack provided in the embodiments of the present application are introduced in detail above. Specific examples are used in the present application to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only used to help understand the technical solutions and core ideas of the present application. Ordinary technicians in this field should understand that they can still modify the technical solutions recorded in the aforementioned embodiments, or replace some of the technical features therein with equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

A secondary battery, comprising a positive electrode plate, an electrolyte and a separator, characterized in that it also comprises:

A negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, the negative electrode active material layer comprising a negative electrode active material, and the negative electrode active material comprising graphite;

Wherein, the non-Faraday specific capacitance of the negative electrode plate is Cdl nF/g, 50≤Cdl≤250.
The secondary battery according to claim 1, characterized in that the resistance of the negative electrode active material layer is R mΩ, 4≤0.1×Cdl-R≤24.
The secondary battery according to any one of claims 1-2 is characterized in that the value range of R is 1≤R≤15.
The secondary battery according to any one of claims 1 to 3, characterized in that the porosity P of the negative electrode sheet is 20% to 40%.
The secondary battery according to any one of claims 1 to 4, characterized in that the coating weight of the negative electrode active material layer on one surface of the negative electrode current collector is CW mg/cm 2 , and 7≤CW≤12.
The secondary battery according to any one of claims 1 to 5, characterized in that the bulk density of the negative electrode active material is 1 g/cm 3 to 2.5 g/cm 3 .
The secondary battery according to any one of claims 1 to 6, characterized in that the particle dispersion degree of the negative electrode active material is 1.5~5.
The secondary battery according to any one of claims 1 to 7, characterized in that the electrolyte includes a sulfur-containing additive, and the sulfur-containing additive includes at least one of formulas (1) to (5):

Formula 1); Formula (2); Formula (3);

Formula (4); Formula (5).
The secondary battery according to claim 8, characterized in that, based on the mass of the electrolyte, the content of the sulfur-containing additive is A%, 0.05≤Cdl×A%≤12.5.
The secondary battery according to any one of claims 8 to 9, characterized in that, based on the mass of the electrolyte, the content of the sulfur-containing additive is A%, and 0.01≤A≤5.
A battery pack, comprising the secondary battery according to any one of claims 1 to 10.